MTB suspension is a huge subject! I will follow up with more detailed information on increasingly complex subjects but for now check out the Basics descriptions below.
From sag to compression damping I will cover things to help you understand what riders are talking about when discussing MTB suspension. If you want some further reading then check out the post on MTB Suspension Setup to see how we make changes to your bike on coaching days.
Sag is expressed as a percentage of overall travel. The amount the suspension compressed when you, the rider, stands on the bike.
For a given air pressure, a rider weighing 100kg may compress a bike’s suspension through 30% of it’s travel. On the same bike with the same air pressure a rider weighing 75kg may compress the suspension to 20% of it’s travel.
That’s why different riders, on different bikes with different riding styles run their sag at different percentages. As a general rule, sag shouldn’t exceed 30% or be less than 15%.
Stroke is the distance moving parts of a fork or shock move between fully extended and full compressed. In a fork this is the same as travel, a 1:1 ratio, so a 160mm travel fork also has a stroke of 160mm.
In a rear shock the stroke figure is a lot smaller but it’s multiplied by the frame’s leverage ratio. So a frame with a shock that has a 65mm stroke and 2:1 leverage ratio will have 130mm of travel.
An air spring is a volume of air sealed inside a chamber set to a specific pressure. Increasing the amount of air pumped into this chamber increases the pressure.
The more air within that chamber, the higher the pressure and the harder it is to compress for a given force.
A heavier rider will add more pressure to support their weight and a lighter rider will require less. Both riders need to find the happy medium, so their fork or shock can perform it’s compression and rebound duties effectively.
A Coil Spring is a more recognisable form of spring and performs exactly the same task as an air spring. Normally made from a steel or sometimes titanium alloy a coil spring is round bar wound to form the coils of a spring.
The diameter of the profile used to make the spring, the number of coils and material it’s made from define the rate of the spring.
Both air and coil springs have a spring rate, but they are defined differently in each case.
A coil spring rate is defined as the weight needed to compress that spring one inch. This is counted in pounds in MTB suspension and is (almost) exclusively linear.
Linear means that the same force is required throughout the compression of a spring to compress through it’s stroke, remember stroke?
An air spring works differently. As the air spring is compressed the volume of the chamber holding the air gets smaller. Less space for the air equals an increase in pressure. This makes compressing that spring PROGRESSIVELY harder through it’s stroke, so an air shock is progressive.